1. Field of the Invention
The present invention relates to a semiconductor device in which a fine copper wire (Cu wire) is used and a die bonding material is used for bonding a semiconductor chip on a die pad. The present invention also relates to a method of manufacturing a semiconductor device with which a copper (Cu) ball is brought into contact with an aluminum (Al) electrode pad for a predetermined time and the Cu ball is compressed.
2. Description of the Related Art
FIG. 1 is a cross-sectional side view of a conventional semiconductor device. In the drawing, a semiconductor chip 1 is bonded onto a die pad 3 of a lead frame with an epoxy resin 4. An Al electrode pad 5 is provided on the surface of the semiconductor chip 1, the surface of the portion without the Al electrode pad 5 being covered with an SiO.sub.2 glass coating 6 for preventing corrosion of the Al wiring by impurities in the molding resin of the chip 1. The Al electrode 5 and an inner end of lead 7 are electrically connected to each other with a Cu wire 8. Silver is plated on the surface of the inner lead 7 and the die pad 3. Both are formed of a copper alloy or an iron nickel alloy.
FIG. 2 is a cross-sectional side view of another conventional semiconductor device which is the same as that shown in FIG. 1 except that gold-silicon (Au-Si) solder 10 is used as a die bonding material.
In the conventional semiconductor devices described above, it is generally necessary to select a suitable die bonding material. An unsuitable material causes degradation of the semiconductor chip during heating in the die bonding process and junction faults during the wire bonding process.
When the die bonding material is an epoxy resin 4 (FIG. 1), the epoxy resin 4 is cured at a temperature of 150.degree. C. to 250.degree. C. which does not degrade the semiconductor chip 1. However, some problems occur in the wire bonding process.
FIGS. 3A and 3B are respectively a plan view and a cross-sectional view of the Al electrode pad 5 of the semiconductor chip 1 bonded on the die pad 3 with the epoxy resin 4 after the Al electrode pad 5 has been removed by etching with nitric acid (HNO.sub.3) following bonding of a Cu ball to the Al electrode pad 5. In these drawings, the Al leaves an exuded Al portion 11. A SiO.sub.2 film 13, which is under Al film 12, is thus exposed. As a result, the high temperature storage life deteriorates, as described in Japanese Published Patent Application 1-143332. If ultrasonic energy which causes the exudation of Al is decreased, no Cu-Al alloy layer is created so that high temperature storage life deteriorates.
The reasons for these effects are the estimated wire bonding temperature is 250.degree. C. to 300.degree. C. while the glass transition temperature Tg of the epoxy resin is 110.degree. C. to 150.degree. C. The Cu ball is plastically deformed on the Al electrode pad 5 by applying a greater ultrasonic vibration energy than in the case of an Au ball. Therefore, more work hardening occurs with the Cu ball than with an Au ball.
When the die bonding material is the Au-Si solder 10 (FIG. 2), the die bonding temperature is higher than 370.degree. C. which is the liquidus temperature of the Au-Si solder. This causes cracks in the glass coating 6 and degrades the semiconductor chip 1.
FIG. 4A is a perspective view of the semiconductor chip 1 and the die pad 3, and FIG. 4B is a cross-sectional side view taken along line A--A of FIG. 4A. FIG. 5 is a plan view of the semiconductor chip 1 on which the structural analysis is carried out. In the drawings, Al wiring 14 is entirely covered with the glass coating 6. In the structural analysis, after die bonding, cracks were found in the glass coating 6 of the semiconductor chip 1 bonded on the die pad 3 with the Au-Si solder 10. In the analysis, the chip was immersed in a solution of 80.degree. C. to 90.degree. C. phosphoric acid (H.sub.3 PO.sub.4) for 20 minutes, and the Al wiring 14 under the glass coating 6 was etched. A resin molded chip deteriorated in pressure cooker tests after the resin deteriorated. The reason for the cracking is believed to be the concentration of stress in a certain portion of the glass coating 6 over the Al wiring 14 because the coefficients of thermal expansion of the glass coating 6 and the semiconductor chip 1 are different (SiO.sub.2 of the glass coating: 0.65.times.10.sup.6 /.degree. C., Si of the semiconductor chip: 3.5.times.10.sup.6 /.degree. C.). When the die bonding material is Au-Si solder 10, the wire bonding temperature must be raised to a higher temperature than that of an Au wire to encourage mutual diffusion between the Cu wire and the Al electrode.
In the above-described semiconductor device, when the die bonding material is an epoxy resin, the semiconductor chip is not bonded well because the temperature during wire bonding is higher than that of the glass transition temperature of the epoxy resin. In addition, the Cu ball is plastically deformed on the Al electrode pad because more ultrasonic vibration energy is applied to a Cu ball than for an Au ball because work hardening of the Cu ball is higher than that of the Au ball. This hardening pushes Al out of the Al electrode pad the SiO.sub.2 under film is thus exposed, and the semiconductor device deteriorates in a high temperature storage test.
When Au-Si solder is used for the purpose of establishing a wire bonding temperature higher than that used with an Au wire, the glass coating of the semiconductor chip cracks at the die bonding temperature, and a resin bonded chip deteriorates in the pressure cooker test.